Lowered Source/Drain Transistors

ABSTRACT

A novel transistor structure and method for fabrication the same. The novel transistor structure comprises first and second source/drain (S/D) regions whose top surfaces are lower than a top surface of the channel region of the transistor structure. The method for fabricating the transistor structure starts out with a planar semiconductor layer and a gate stack on top of the semiconductor layer. Then, top regions of the semiconductor layer on opposing sides of the gate stack are removed. Then, regions beneath the removed regions are doped to form lowered S/D regions of the transistor structure

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to semiconductor transistors, and more particularly, to lowered source/drain semiconductor transistors.

2. Related Art

A typical semiconductor transistor comprises a channel region and first and second source/drain (S/D) regions formed in a semiconductor layer, wherein the channel region is disposed between the first and second S/D regions. The typical semiconductor transistor further comprises a gate stack (that includes a gate dielectric region directly on top the channel region and a gate region on top of the gate dielectric region) directly above the channel region. In addition, first and second gate spacers are formed on sidewalls of the gate stack so as to define the first and second S/D regions, respectively. The capacitance between the gate region and the first S/D region has several components one of which is defined by a path from the gate region to the first S/D region through the first gate spacer. This capacitance component is usually referred to as the out-fringing capacitance. For example, the out-fringing capacitance between the gate region and the second S/D region is defined by a path from the gate region to the second S/D region through the second gate spacer.

It is desirable to minimize the out-fringing capacitances between the gate region and the first and second S/D regions in order to increase transistor performance or to reduce transistor switching time. Therefore, there is a need for a novel transistor structure in which the out-fringing capacitances between the gate region and the first and second S/D regions are relatively less than those of the prior art. There is also a need for a method for fabricating the novel transistor structure.

SUMMARY OF THE INVENTION

The present invention provides a semiconductor structure, comprising (a) a semiconductor layer including a channel region and first and second source/drain regions, wherein the channel region is disposed between the first and second source/drain regions, and wherein top surfaces of the first and second source/drain regions are below a top surface of the channel region; (b) a gate dielectric region on the channel region; and (c) a gate region on the gate dielectric region, wherein the gate region is electrically isolated from the channel region by the gate dielectric region.

The present invention also provides a method for fabricating a semiconductor structure, the method comprising the steps of (a) providing a semiconductor layer and a gate stack on the semiconductor layer, wherein the semiconductor layer comprises (i) a channel region directly beneath the gate stack and (ii) first and second semiconductor regions essentially not covered by the gate stack, and wherein the channel region is disposed between the first and second semiconductor regions; (b) removing the first and second semiconductor regions; and (c) doping regions directly beneath the removed first and second semiconductor regions so as to form first and second source/drain regions, respectively, such that top surfaces of the first and second source/drain regions are below a top surface of the channel region.

The present invention also provides a method for fabricating a semiconductor structure, the method comprising the steps of (a) providing (i) an underlying dielectric layer, (ii) a semiconductor layer on the underlying dielectric layer, and (iii) a gate stack on the semiconductor layer; (b) implanting first dopants in a top layer of the underlying dielectric layer except in a separating dielectric region of the top layer directly beneath the gate stack; (c) removing the top layer of the underlying dielectric layer except the separating dielectric region; (d) epitaxially growing semiconductor regions to fill the removed top layer of the underlying dielectric layer; and (e) implanting second dopants in semiconductor regions of the semiconductor layer and the epitaxially grown semiconductor regions on opposing sides of the gate stack so as to form first and second source/drain regions such that the separating dielectric region is disposed between the first and second source/drain regions.

The present invention provides a novel transistor structure in which the out-fringing capacitances between the gate region and the first and second S/D regions are relatively less than those of the prior art. The present invention also provides a method for fabricating the novel transistor structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1K show cross-section views of a semiconductor structure used to illustrate a method of fabricating semiconductor structures, in accordance with embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A-1K show cross-section views of a semiconductor structure 100 used to illustrate a method of fabricating semiconductor structures, in accordance with embodiments of the present invention. More specifically, with reference to FIG. 1A, in one embodiment, the method starts out with an SOI (silicon on insulator) substrate 110 comprising, illustratively, a silicon layer 110 a, an underlying dielectric layer 110 b (usually referred to as BOX, i.e., buried oxide layer) on top of the silicon layer 110 a, and another silicon layer 110 c on top of the underlying dielectric layer 110 b. Starting from FIG. 1B, the silicon layer 110 a is omitted for simplicity.

Next, in one embodiment, the method comprises the step of forming a gate dielectric layer 120 on top of the silicon layer 110 c. In one embodiment, the gate dielectric layer 120 can comprise silicon dioxide and can be formed by thermally oxidizing a top surface 112 of the silicon layer 110 c.

Next, in one embodiment, a gate layer 130 is formed on top of the gate dielectric layer 120. In one embodiment, the gate layer 130 can comprise poly-silicon. Next, in one embodiment, a hard mask dielectric layer 140 is formed on top of the poly-silicon layer 130. In one embodiment, hard mask dielectric layer 140 can comprise silicon dioxide and can be formed by, illustratively, chemical vapor deposition (i.e., CVD). Then, in one embodiment, a photoresist layer 150 is formed on top of the hard mask dielectric layer 140.

Next, in one embodiment, the photoresist layer 150 is patterned to become the patterned photoresist layer 150′ by, illustratively, photolithography (i.e., the regions of the photoresist layer 150 represented by the dashed lines are removed).

Next, in one embodiment, the patterned photoresist layer 150′ can be used as a mask to etch the hard mask dielectric layer 140 and then the gate layer 130 so as to form the hard mask dielectric region 140′ and the gate region 130′, respectively. In other words, the regions of the hard mask dielectric layer 140 and the gate layer 130 represented by the dashed lines are removed.

Next, with reference to FIG. 1B, in one embodiment, the method proceeds with an implantation step of using the regions 130′, 140′, and 150′ as a mask to implant nitrogen in a top layer 114 of the underlying dielectric layer 110 b. As a result, regions 114 a and 114 b of the top layer 114 are doped with nitrogen except for a separating dielectric region 114 c directly beneath the regions 130′, 140′, and 150′. In general, any dopants can be used here instead of nitrogen provided that the doped regions 114 a and 114 b doped with the dopants can be later etched away (i.e., removed) essentially without affecting the other regions of the underlying dielectric layer 110 b.

Next, with reference to FIG. 1C, in one embodiment, the patterned photoresist layer 150′ (FIG. 1B) can be removed, and a nitride layer 150 can be blanket-deposited on top of the structure 100.

Next, with reference to FIG. 1D, in one embodiment, the method can proceed with an anisotropic etching step that removes most of the nitride layer 150 and leaves nitride spacers 150 a and 150 b on sidewalls of the gate stack 130′,140′ (that comprises the gate region 130′ and the hard mask dielectric region 140′). In one embodiment, the anisotropic etching step can be RIE (Reactive Ion Etching). Next, in one embodiment, an oxide (e.g., SiO₂) layer 160 can be blanket-deposited on the structure 100 by, illustratively, CVD.

Next, in one embodiment, the gate stack 130′,140′ can be used as a mask to implant germanium in a top layer 116 of the silicon layer 110 c. As a result, doped regions 116 a and 116 b of the top layer 116 are doped with germanium except for a region 116 c directly beneath the gate stack 130′,140′. In general, any dopants can be used here instead of germanium provided that the resulting silicon regions 116 a and 116 b doped with the dopants can be later etched away essentially without affecting the other regions of the silicon layer 110 c.

Next, with reference to FIG. 1E, in one embodiment, nitride spacers 170 a and 170 b can be formed on sidewalls of the gate stack 130′,140′ (that now includes a portion of the oxide layer 160 that covers the gate stack 130′,140′). In one embodiment, the nitride spacers 170 a and 170 b can be formed by blanket-depositing a nitride layer (not shown) on top of the structure 100 and then etching back.

Next, in one embodiment, the method proceeds with an implantation step (represented by arrow 117 a′) of implanting germanium in the silicon layer 110 c at an angle such that the resulting doped region 117 a is deeper than the doped region 116 a and extends under the nitride spacer 170 a. Then, in one embodiment, the method proceeds with an implantation step (represented by arrow 117 b′) of implanting germanium in the silicon layer 110 c at an angle such that the resulting doped region 117 b is deeper than the doped region 116 b and extends under the nitride spacer 170 b. The arrows 117 a′ and 117 b′ also indicate the respective directions of germanium bombardments.

Next, in one embodiment, the method proceeds with an implantation step (represented by arrow 118) of implanting germanium vertically in the silicon layer 110 c such that the resulting doped regions 118 a and 118 b are deeper than the doped regions 117 a and 117 b, respectively. The arrow 118 also indicates the direction of germanium bombardment. Starting from FIG. 1F, the doped regions 116 a, 117 a, and 118 a are collectively referred to as the doped region 119 a. Similarly, the doped regions 116 b, 117 b, and 118 b are collectively referred to as the doped region 119 b.

Next, with reference to FIG. 1F, in one embodiment, oxide spacers 180 a and 180 b are formed on sidewalls of the nitride spacers 170 a and 170 b, respectively. In one embodiment, the oxide spacers 180 a and 180 b can be formed by blanket-depositing an oxide layer (not shown) on top of the structure 100 and then etching back. As a result, a top region of the oxide layer 160 is etched away, and the nitride spacers 160 a and 160 b are exposed to the atmosphere. Also as a result, the doped regions 119 a and 119 b are exposed to the atmosphere. The oxide layer 160 is reduced to the oxide regions 160 a and 160 b. The gate dielectric layer 120 is reduced to gate dielectric region 120′.

Next, with reference to FIG. 1G, in one embodiment, the method proceeds with an etching step of anisotropically etching away (illustratively, using RIE) silicon regions exposed to the atmosphere while leaving essentially intact other regions comprising other materials such as oxide and nitride. As a result, regions 119 a and 119 b of the silicon layer 110 c are removed.

Next, in one embodiment, the nitrogen-doped regions 114 a and 114 b can be removed by a wet-etching process which essentially affects only nitrogen-doped oxide material and essentially does not affect other materials such as nitride, silicon, and undoped oxide.

Next, with reference to FIG. 1H, in one embodiment, silicon is epitaxially grown from the silicon layer 110 c (including the doped regions 119 a and 119 b) to top surfaces 192 a and 192 b.

Next, in one embodiment, the resulting silicon layer 110 c is anisotropically etched back (illustratively, using RIE) to top surfaces 194 a and 194 b, respectively. In one embodiment, the top surfaces 194 a and 194 b of the resulting silicon layer 110 c after etching back are below the bottom surfaces 195 a and 195 b (FIG. 1G) of the germanium-doped regions 119 a and 119 b, respectively.

Next, in one embodiment, an anneal process can be performed to diffuse germanium in the germanium-doped regions 119 a and 119 b into the silicon layer 110 c.

Next, with reference to FIG. 1I, in one embodiment, the germanium-doped regions 119 a and 119 b (FIG. 1H) of the silicon layer 110 c can be removed (illustratively, by wet etching) while leaving essentially intact other regions of the silicon layer 110 c that are not doped with germanium.

Next, in one embodiment, an S/D implantation step can be performed to form S/D regions 210 a and 210 b in the silicon layer 110 c. In one embodiment, an S/D anneal step can be performed after the S/D implantation step.

Next, with reference to FIG. 1J, in one embodiment, the method proceeds with a step of anisotropically etching (illustratively, using RIE) the exposed nitride regions 150 a, 150 b, 170 a, and 170 b (FIG. 1I). As a result, the nitride spacers 170 a and 170 b are removed. The nitride regions 150 a and 150 b are thin and protected by surrounding oxide regions 160 a, 160 b, and 140′ (FIG. 1I). As a result, the etch rate for the nitride regions 150 a and 150 b is much slower than that for the nitride spacers 170 a and 170 b. Therefore, when the nitride spacers 170 a and 170 b are completely removed, the nitride regions 150 a and 150 b can be almost intact.

Next, in one embodiment, the method proceeds with a step of anisotropically etching (illustratively, using RIE) the exposed oxide regions 160 a, 160 b, and 140′ (FIG. 1I). As a result, the hard mask dielectric region 140′ is removed, while the oxide regions 160 a and 160 b are reduced to the oxide spacers 160 a′ and 160 b′, respectively.

Next, in one embodiment, a halo implantation step (represented by an arrow 220 a′) can be performed to form a halo region 220 a. Next, in one embodiment, another halo implantation step (represented by an arrow 220 b′) can be performed to form a halo region 220 b. The arrows 220 a′ and 220 b′ also indicate the respective directions of halo ion bombardments.

Next, in one embodiment, an extension implantation step (represented by arrows 230) can be performed to form extension regions 230 a and 230 b. The arrow 230 also indicates the direction of extension ion bombardments.

Next, in one embodiment, a halo and extension anneal step can be performed to anneal the resulting halo regions 220 a and 220 b and the resulting extension regions 230 a and 230 b.

Next, with reference to FIG. 1K, in one embodiment, oxide spacers 240 a and 240 b are formed on sidewalls of the oxide spacers 160 a′ and 160 b′, respectively. In one embodiment, the oxide spacers 240 a and 240 b can be formed by blanket-depositing an oxide layer (not shown) on top of the structure 100 and then etching back. Now, the gate region 130′ and the gate dielectric region 120′ can be collectively referred to as the gate stack 120′,130′ of the structure 100.

In summary, the method for forming lowered S/D transistor 100 starts out with a planar silicon layer 110 c (FIG. 1A). Then, the silicon regions 119 a and 119 b (FIG. 1H) are doped with germanium so that they can be removed later (FIG. 1I) without affecting other silicon regions of the silicon layer 110 c. As a result, the transistor 100 (FIG. 1K) has lowered S/D regions 210 a and 210 b (i.e., top surfaces 212 a and 212 b of the S/D regions 210 a and 210 b, respectively, are lower than a top surface 242 of the channel region 240). Considering a path from the gate region 130′ to the S/D region 210 a through the nitride spacer 150 a, the oxide spacers 160 a′ and 240 a, because of the lowered S/D region 210 a, the path is extended when it goes through the oxide spacer 240 a. As a result, the out-fringing capacitance between the gate region 130′ to the S/D region 210 a is reduced. For a similar reason, the out-fringing capacitance between the gate region 130′ and the S/D region 210 b is also reduced.

To form the separating dielectric region 114 c (FIG. 1C), the oxide regions 114 a and 114 b of the underlying dielectric layer 110 b are doped with nitrogen so that the oxide regions 114 a and 114 b can be later removed (FIG. 1G) and replaced by epi-silicon (epi=epitaxially grown) as shown in FIG. 1H. As a result, the separating dielectric region 114 c is disposed between the S/D regions 210 a and 210 b (FIG. 1K). Because of the separating dielectric region 114 c, the channel region 240 (immediately beneath the gate dielectric region 120′) is thinner. As a result, short channel effects are improved.

While particular embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention. 

1. A semiconductor structure, comprising: (a) a semiconductor layer including a channel region and first and second source/drain regions, wherein the channel region is disposed between the first and second source/drain regions, and wherein top surfaces of the first and second source/drain regions are below a top surface of the channel region; (b) a gate dielectric region on the channel region; and (c) a gate region on the gate dielectric region, wherein the gate region is electrically isolated from the channel region by the gate dielectric region.
 2. The structure of claim 1, further comprising an underlying dielectric layer beneath and in direct physical contact with the semiconductor layer, wherein the underlying dielectric layer is directly beneath the channel region and the first and second source/drain regions.
 3. The structure of claim 2, wherein the underlying dielectric layer comprises a separating dielectric region disposed between the first and second source/drain regions.
 4. The structure of claim 3, wherein the separating dielectric region is directly beneath the channel region.
 5. A method for fabricating a semiconductor structure, the method comprising the steps of: (a) providing a semiconductor layer and a gate stack on the semiconductor layer, wherein the semiconductor layer comprises (i) a channel region directly beneath the gate stack and (ii) first and second semiconductor regions essentially not covered by the gate stack, and wherein the channel region is disposed between the first and second semiconductor regions; (b) removing the first and second semiconductor regions; and (c) doping regions directly beneath the removed first and second semiconductor regions so as to form first and second source/drain regions, respectively, such that top surfaces of the first and second source/drain regions are below a top surface of the channel region.
 6. The method of claim 5, wherein the gate stack comprises (i) a gate dielectric region on the semiconductor layer and (ii) a gate region on the gate dielectric region, wherein the gate region is electrically isolated from the channel region by the gate dielectric region.
 7. The method of claim 5, wherein the step of removing the first and second semiconductor regions comprises the steps of: doping the first and second semiconductor regions of the semiconductor layer with first dopants; and etching away the first and second semiconductor regions but leaving essentially intact the gate stack and regions of the semiconductor layer which are not doped with the first dopants.
 8. The method of claim 7, wherein the step of doping the first and second semiconductor regions with the first dopants is performed such that each region of the first and second semiconductor regions of the semiconductor layer has a thickness in a first direction perpendicular to a top surface of the channel region that increases when moving in a second direction away from the gate stack, and wherein the second direction is parallel to the top surface of the channel region.
 9. The method of claim 8, wherein the step of doping the first and second semiconductor regions with the first dopants further comprises the step of forming spacers on sidewalls of the gate stack.
 10. The method of claim 5, wherein the semiconductor layer comprises silicon, and wherein the first dopants in the first and second semiconductor regions comprise germanium.
 11. The method of claim 5, further comprising the steps of: (α) providing an underlying dielectric layer wherein the semiconductor layer is on the underlying dielectric layer; (β) implanting second dopants in a top layer of the underlying dielectric layer except in a separating dielectric region of the top layer directly beneath the gate stack; (γ) etching away the top layer of the underlying dielectric layer except the separating dielectric region; and (δ) epitaxially growing a semiconductor material to fill the removed top layer of the underlying dielectric layer before the step (b) is performed.
 12. The method of claim 11, wherein step (β) comprises the step of using the gate stack as a mask to implant the second dopants in the top layer of the underlying dielectric layer except in the separating dielectric region, and wherein step (γ) is performed such that regions of the underlying dielectric layer which are not doped with the second dopants are essentially intact.
 13. The method of claim 12, wherein the second dopants comprise nitrogen, and wherein the underlying dielectric layer comprises silicon dioxide.
 14. The method of claim 11, wherein the step (δ) is performed until a top surface of epitaxially grown semiconductor regions is at a level lower than a bottom surface of the first and second semiconductor regions.
 15. A method for fabricating a semiconductor structure, the method comprising the steps of: (a) providing (i) an underlying dielectric layer, (ii) a semiconductor layer on the underlying dielectric layer, and (iii) a gate stack on the semiconductor layer; (b) implanting first dopants in a top layer of the underlying dielectric layer except in a separating dielectric region of the top layer directly beneath the gate stack; (c) removing the top layer of the underlying dielectric layer except the separating dielectric region; (d) epitaxially growing semiconductor regions to fill the removed top layer of the underlying dielectric layer; and (e) implanting second dopants in semiconductor regions of the semiconductor layer and the epitaxially grown semiconductor regions on opposing sides of the gate stack so as to form first and second source/drain regions such that the separating dielectric region is disposed between the first and second source/drain regions.
 16. The method of claim 15, wherein step (b) comprises the step of using the gate stack as a mask to implant the first dopants in the top layer of the underlying dielectric layer except in the separating dielectric region.
 17. The method of claim 15, wherein step (c) comprises a wet-etching process that etches away regions of the underlying dielectric layer doped with the first dopants but leaves essentially intact regions of the underlying dielectric layer not doped with the first dopants.
 18. The method of claim 17, wherein step (c) further comprises the step of, before the wet-etching process is performed, removing regions of the semiconductor layer so as to expose regions of the underlying dielectric layer that are doped with the first dopants.
 19. The method of claim 18, wherein the step of removing the regions of the semiconductor layer so as to expose the regions of the underlying dielectric layer that are doped with the first dopants comprises the steps of: forming gate spacers on side walls of the gate stack; and using the gate stack and the gate spacers as a mask to RIE (reactive ion etching) away the regions of the semiconductor layer so as to expose the regions of the underlying dielectric layer that are doped with the first dopants.
 20. The method of claim 15, wherein the first dopants comprise nitrogen, and wherein the underlying dielectric layer comprises silicon dioxide. 